Code excitation linear predictive (CELP) encoder and decoder and code excitation linear predictive coding method

ABSTRACT

There is provided a code excitation linear predictive (CELP) coding or decoding apparatus in which a code vector, which is transmitted by a codebook such as a stochastic codebook, is converted adaptively in accordance with vocal tract analysis information (LPC) so that a high quality reproduction speech is obtained at a low coding rate. Further, in order to obtain a similar effect, a pulse-like excitation codebook formed of an isolated impulse is provided in addition to the adaptive excitation codebook and stochastic excitation codebook so that either the stochastic excitation codebook or the pulse-like excitation codebook is selectively used to provide a vocal tract parameter as a linear spectrum pair parameter.

TECHNICAL FIELD OF THE INVENTION

This invention relates to an encoder and a decoder based on the codeexcitation linear predictive coding (CELP) system.

BACKGROUND OF THE INVENTION

Conventionally, as a highly efficient coding system for a speech signalincluding an audible signal in the field of digital transportablecommunication systems, code excitation linear predictive coding and amodification, thereof have been used. The modification is a vector sumexcitation linear predictive coding system (VSELP). The coding apparatuswhich uses the code excitation linear predictive coding (CELP) isdisclosed in, for example, N. S. Jayant and J. H. Chen, "Speech Codingwith Time-varying Bit Allocation to Excitation and LPC Parameters",Proc. ICASSP, pp 65-68, 1989.

A fundamental construction of a coding system relative to the speechsignal obtains vocal tract parameters representing vocal tractproperties and excitation source parameters representing excitationsource information. In a recent CELP system, an excited signal as anexcitation source information is encoded by means of both adaptiveexcitation code vectors, which contribute to a stochastically strongerperiodic excitation signal, and stochastic excitation code vectors whichcontribute to a stochastic less periodic random excitation signal. Thenthe coded excitation signals are stored in a codebook, and optimumadaptive excitation code vectors and stochastic excitation code vectorsare located in each codebook so that a weighted error power sum betweenan input speech vector and a synthetic speech vector becomes minimum.Then, whatever it is of a forward-type coding system which obtains vocaltract parameters from an input speech vector or of a backward-typecoding system which obtains vocal tract parameters from synthetic speechvectors, at least the excitation source parameters, that is, adaptiveexcitation code and stochastic excitation code information aretransmitted.

By utilizing the code excitation linear predictive (CELP) system asdescribed above, it is known that high quality regenerated speechsignals are obtained at a coding rate of 6 kbit/s to 8 kbit/s.

However, some communication systems require lower coding rate, forexample 4 kbit/s or less. In such a lower coding rate, regardless ofwhatever the forward type which transmits both vocal tract parametersand excitation source parameters or the backward type which transmitsexcitation source parameters is used, the number of coded bits which areassigned to the excitation source parameters is smaller and the numberof adaptive excitation code vectors stored in the adaptive excitationcodebook and the number of stochastic excitation code vectors stored inthe stochastic excited codebook become smaller. Consequently, thequality of the regenerated speech signal inevitably degrades at thelower coding rate as described above.

Besides, the adaptive excited codebook is adaptively renewed bysynthetic code vectors of the optimum adaptive excitation code vectorsand stochastic excitation code vectors and, accordingly, it can bedetermined that the adaptive excitation code vectors are formed on thebasis of the stochastic excitation code vectors. Therefore, the currentCELP coding has a poor tracking capability for a voice signal having anature of strong periodicity. Consequently, the generated speech signallacks clearness.

SUMMARY OF THE INVENTION

The present invention is based upon the foregoing problems and an objectof the present invention is to provide a code excitation linearpredictive coding encoder and decoder which can provide a high qualityregenerated speech signal even when pulse-like noise components arecontained in the input speech vectors.

Another object of the present invention is to provide a code excitationlinear predictive coding encoder and decoder which can provide ahigh-quality regenerated speech signal even when a lower coding rate isemployed.

According to the present invention, there is provided a code excitationlinear predictive coding apparatus which uses, as a speech excitationsource information, excitation signals in the form of an excitationcodebook, wherein the apparatus is provided with a code vectorconversion circuit which converts the frequency characteristics of fixedcode vector such as stochastic excitation code vector transmitted fromthe excitation codebook into the predetermined frequency characteristicsat the time of output of the excitation code vectors. A primary reasonfor providing the code vector conversion circuit is set forth below.Conventionally, the frequency characteristic of an excitation signal ismodelled as "theoretically white" and yet it actually is not "white" butis recognized by examinations to have a characteristic which is close tothe frequency characteristic of input speech vectors. Therefore, thecloser the fixed code vector frequency characteristic is set to thefrequency characteristics of the input speech vectors, the higher thequality of the synthetic speech vector obtained and, moreover, theeffective frequency component of the excitation code vectors becomesmuch larger than the quantization error vectors so that a masking effectof the quantization error vectors can be obtained. As an informationrepresenting frequency characteristic of the code conversion circuit,parameters of LPC (linear predictive coefficient) and optimum adaptiveexcitation code information which means pitch predictive information(which includes VQ gains) are used. Thus, the code vector conversioncircuit controls the frequency characteristics of the stochasticexcitation code vectors and so forth, in accordance with thisinformation.

Further, in the present invention, there is provided a code excitationlinear predictive decoding apparatus which has a code vector conversioncircuit which forces the fixed code vector frequency characteristicsclose to the input speech vector frequency characteristic in accordancewith the respective code excitation linear predictive coding system.

In the code vector converter circuit, an impulse response is determinedby the following formula (1) as a filter transfer function H(Z)according to the vocal tract parameters,

    H(Z)=(1-ΣA.sup.j ajZ.sup.-j)/(1-ΣB.sup.j aj.sup.-j)(1)

or as an impulse response determined by the following formula (2) inaccordance with an excited pitch lag,

    H(z)=1/(1-εZ.sup.-L)                               (2)

or as an impulse response which is a cascade-connected filterrepresented by formulas (1) and (2) used to provide a convolutiontreatment to the stochastic excitation code vectors. Thereafter adaptiveexcitations code vectors are added to produce excitation code vectors.Here, aj(j=1 to p) represents a parameter of LPC and p represents theorder of LPC analysis. A, B and ε are constants which are determined inthe range of 0<A<1, 0<B<1 and 0<ε≦1, respectively, and L represents apitch lag.

Further, the present invention provides a code excitation linearpredictive coding or decoding apparatus which is provided, as anexcitation codebook, with an adaptive excitation codebook and stochasticexcitation codebook, in which a pulse-like excitation codebook storing apulse-like excitation code vector which consists of an isolated impulsein addition to the adaptive excitation codebook and stochasticexcitation codebook is provided so that the current CELP coding has goodtracking capability for a speech signal having strong periodicity. Thus,a clear regenerated speech signal can be obtained.

Further, in the code excited linear predictive coding apparatus,excitation code vectors from the stochastic excitation codebook orpulse-like excitation codebook are selectively used, and this selectedinformation is transmitted to the code excitation linear predictivedecoder apparatus. In this code excitation linear predictive decoderapparatus, the excitation code vectors from the stochastic excitationcodebook or pulse-like excitation codebook are selected in accordancewith the information transmitted from the code excitation linearpredictive coding apparatus.

In addition, in each of the above-described code excitation linearpredictive encoders, the output of vocal tract parameters are assignedto be LSP (linear spectral pair) parameters and these linear spectralpair parameters are utilized for speech regeneration in the codeexcitation linear predictive decoder so that the regeneration speechquality at the lower coding rate can be improved from the viewpoint ofvocal tract parameters. The reasons for using LSP parameters as thevocal tract parameters are that the interpolation characteristicsrelative to the frequency characteristics of the vocal tract areimproved, that the LSP parameters provides less distortion to the vocaltract spectral than LPC parameters even when the LSP parameters arecoded by a smaller number of code bits, and that effective coding can beobtained by the combination with vector quantization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a code excitation linear predictive encoder(coding apparatus) according to first and second embodiments of thepresent invention. The first and second embodiments of the encoder shownin FIG. 1 differ from the prior art only in that a code vectorconversion circuit (109) has been added.

FIG. 2 is a block diagram of a code excitation linear predictive decoderin correspondence with the code excitation linear predictive encodershown in FIG. 1. The decoder shown in FIG. 2 differs from the prior artonly in that a code vector conversion circuit (206) has been added.

FIG. 3 is a block diagram of a code excitation linear predictive encoder(coding apparatus) according to a third embodiment of the invention, thesolid lines between components in FIGS. 1-3 representing the flow ofsignals in the encoding and decoding apparatus and the dashed linesrepresenting the flow of information comprising the indices of the codebooks. The encoder of FIG. 3 differs from the prior art only in that apulse-like excitation code book (322), a fixed codebook selection switch(326) and a code vector conversion circuit (328) have been added.

FIG. 4 is a block diagram of a code excitation linear predictive decoderin correspondence with the code excitation linear predictive encodershown in FIG. 3. The decoder of FIG. 4 differs from the prior art onlyin that a pulse-like excitation codebook (445), a fixed codebookselection switch (448) and a code vector conversion circuit (450) havebeen added.

FIG. 5 is a detailed block diagram of the code vector conversioncircuits shown in FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the code excitation linear predictive codingapparatus (encoder) and the code excitation linear predictive decodingapparatus (decoder) according to the present invention will be describedwith reference to the figures attached herewith.

Referring to FIG. 1 which shows a code excitation linear predictiveencoder (coding apparatus) according to a first embodiment of thepresent invention, an input speech vector S which has been input in eachframe from an input terminal 101 is first transmitted to a vocal tractanalysis circuit 102 to obtain a vocal tract parameter a_(j) (linearpredictive coefficient).

An LPC (linear predictive coefficient) quantization circuit 103quantizes vocal tract predictive parameter a_(j) and transmits its codeI_(c) (quantized LPC code) to an LPC inverse-quantization circuit 104and a multiplex circuit 106.

The LPC inverse-quantization circuit 104 serves to convert the LPC codeI_(c) into vocal tract predictive parameter a_(qj) and transmits thesame to a synthesis filter 105.

Then, an adaptive excitation code vector e_(ai) (i=1 to n) is outputtedfrom an adaptive excitation codebook 107 and similarly, a stochasticexcitation, code vector e_(sl) (l=1 to m) is outputted from a stochasticexcitation codebook 108. Similarly, excitation gains B_(k) and Y_(k)(k=1 to r) are outputted from a VQ gain codebook 110,

A code vector conversion circuit 109, which has an impulse response of afilter transfer function H(Z) represented by the following formula (3),performs convolutional computation with stochastic excitation codevector e_(sl) from stochastic excitation codebook 108, and transmits aconverted stochastic excitation code vector e_(scl). ##EQU1## whereina_(qj) represents an output of LPC inverse quantization circuit 104 andp represents the vocal tract analysis order.

The adaptive excitation code vector e_(ai) is multiplied by the gainB_(k) by means of a multiplier 113 to produce a vector e_(aik) and, onthe other hand, the converted stochastic excitation code vector e_(scl)is multiplied by the gain Y_(k) by means of a multiplier 114 to producea vector e_(sclk).

An adder 115 adds the components of vector e_(aik) and vector e_(sclk)and produces an excitation code vector e.

The synthesis filter 105 calculates synthetic speech vector S_(w)corresponding to the excitation codevector e and transmits it to asubtracter 116.

The subtracter 116 performs the subtraction between the synthesizedspeech vector S_(w) and the input speech vector S, and the obtainederror vector e_(r) between Sw and S is transmitted to a perceptualweighting filter 111.

The perceptual weighting filter 111 transmits a perceptual weightingerror vector e_(w) corresponding to the error vector e_(r) to aperceptual weighting error calculation circuit 112.

The perceptual weighting error calculation circuit 112 calculates a meansquare value of each component of the perceptual weighting error vectore_(w) and determines the excitation code vector (i.e., combination of i,l and k) to minimize the mean square error power of e_(w) for the inputspeech vector at the present time. Indexes I_(a), I_(s) and I_(g) ofeach codebook at this moment are transmitted to each of the adaptiveexcitation codebook 107, stochastic excitation codebook 108, VQ gaincodebook 110 and multiplex circuit 106.

The adaptive excitation codebook 107 outputs an optimum adaptiveexcitation code vector e_(ao) assigned by index I_(a), the stochasticexcitation codebook 108 outputs an optimum stochastic excitation codevector e_(s0) assigned by index I_(s), and the VQ gain codebook 110transmits optimum VQ gains β₀ and γ₀ assigned by index I_(g). Codevector conversion circuit 109 converts the stochastic code vector e_(s0)which has been transmitted from the stochastic excitation codebook 108in accordance with the index I_(s) into an optimum converted stochasticexcitation code vector e_(sc0) and then outputs it to the multiplier114.

An optimum excitation code vector e_(opt) composed of the code vectore_(ao) and e_(sco) and the optimum VC gains β₀ and γ₀ is transmitted tothe adaptive excitation codebook 107 and updates the content of theadaptive excitation codebook 107.

The multiplex circuit 106 multiplexes I_(c), I_(a), I_(s), and I_(g), asa total code C, and transmits it to the receiver through an outputterminal 117.

FIG. 2 is a block diagram of a code excitation linear predictive decodercorresponding to the code excitation linear predictive encoder of FIG.1.

In FIG. 2 the total code C from an input terminal 201 is separated by ademultiplex circuit 212 into LPC code I_(c), adaptive excitation codeindex I_(a), stochastic excitation code index I_(s), and VQ gain codeindex I_(g) and they are transmitted, respectively, to LPC inversequantization circuit 202, adaptive excitation codebook 204, stochasticexcitation codebook 205 and VQ gain codebook 207.

The LPC inverse quantization circuit 202 converts the LPC code I_(c)into vocal tract predictive parameter a_(j) and transmits to a synthesisfilter 203. The adaptive excitation codebook 204 outputs adaptiveexcitation code vector e_(a) assigned by the index I_(a), the stochasticexcitation codebook 205 outputs a stochastic excitation code vectore_(s) assigned by the index I_(s), and VQ gain codebook 207 outputsexcitation gains β and γ, assigned by index I_(g).

A codevector conversion circuit 206 converts the vector e_(s) into avector e_(sc) and outputs it similarly to the output of code vectorconversion circuit 109 of the aforementioned code excitation linearpredictive coding apparatus (encoder) of FIG. 1.

The adaptive excitation code vector e_(a) is multiplied by the gain β bymeans of multiplier 208, and the vector e_(sc) is multiplied by gain γby the means of multiplier 209. These multiplied vector components areadded by adder 210, and final excitation code vector e for synthesisfilter 203 is obtained.

Synthesis filter 203 calculates a synthesized speech vector Scorresponding to the excitation code vector e and outputs it to anoutput terminal 211 At the same time, the content of the adaptiveexcitation codebook 204 is updated by vector e.

The code excitation linear predictive encoder according to a secondembodiment of the invention will be explained by again referring to FIG.1.

This code excitation linear predictive encoder according to a secondembodiment has a similar construction to that of the first embodimentexcept for the operation of the codevector conversion circuit 109 and,therefore, the operational mode of the code vector conversion circuit109 will be explained.

The code vector conversion circuit 109, according to the secondembodiment has an impulse response of a filter transfer function H(Z)shown by the following formula (4) and performs convolutionalcomputation with the vector e_(sl) and results in the vector e_(scl).

    H(Z)=1/(1-εZ.sup.-L)                               (4)

Where ε is ≦1.0, and L is a pitchlag obtained from the index of theadaptive excitation code.

Incidentally, in a shift-type adaptive excitation codebook, the index ofthe adaptive excitation code corresponds with the pitch lag index astabulated below. ##STR1##

The convolutional processing of the aforementioned code excitationlinear predictive coding apparatus (encoder) is represented by thefollowing formula (5), provided that the e_(sl) is an output stochasticexcitation code vector of the stochastic excitation codebook, e_(scl) isa stochastic excitation code vector after the conversion, and h is animpulse response of the conversion circuit.

    e.sub.scl =e.sub.sl X h                                    (5)

wherein:

e_(scl) = x₀, X₁, . . . , X_(n-1) !, e_(sl) = Y₀, Y₁, , , Y_(n-1) !,

h= h₀, h₁, . . . , h_(n-1) !, where the bracket ! is a column vector.

x, y and h are elements, and n is subframe length (or frame length).

A transfer function composed of a vocal tract parameter, or a transferfunction composed of the pitch lag, can be used for the impulse responseof code conversion circuit; alternatively, the two transfer functionscan be cascaded to form the impulse response.

FIG. 3 is a block diagram of a code excitation linear predictive encoderaccording to a third embodiment of the invention. In FIG. 3 this codeexcitation linear predictive encoder is primarily composed of an inputspeech process portion 301, optimum synthesized speech search portion302 and multiplex circuit 303.

The input speech process portion 301 has LSP parameter analysis circuit311, LSP parameter coding circuit 312, LSP parameter decoding circuit313, LPC coefficient conversion circuit 314, perceptual weighting filter315, synthesis filter zero input response generation circuit 316,perceptual weighted filter zero input response generation circuit 317,and subtracters 318 and 319. When an input vector is applied, a speechparameter which is to be transmitted to the decoder is obtained and atarget speech vector for a synthesized speech vector is formed by localreproduction.

In the code excitation linear predictive encoder, digitalized discreteinput speech vector series are stored for the time which corresponds toan analysis frame length for obtaining a vocal tract parameter, and thisanalysis frame length is separated into several subframes and processedby input speech processing portion 301.

The input speech vector is given to the LSP parameter analysis circuit311, analyzed by the LSP analysis circuit 311, and converted to an LSPparameter as a vocal tract parameter. This LSP parameter is coded (forexample, to be vector quantized) by LSP parameter coding circuit 312,given to the multiplex circuit 303 as data 303a, corresponding to LSPparameters I_(c) and transmitted to the code excitation linear decoder.The coded LSP parameter is decoded (vector quantized) by LSP parameterdecoding circuit 313 and converted to LPC by the LPC conversion circuit314. The thus converted LPC is used as a tap coefficient for perceptualweighting filter 315, synthesis filter zero input response generationcircuit 316, perceptual weighted filter zero input generation circuit317 and a synthesis filter 329 which will be described presently, andgiven also to a code vector conversion circuit 328. The quantized LSPparameter is converted into LPC.

Next, an operation for forming a target speech vector, relative to asynthesized speech vector which is locally reproduced from the inputspeech vector, will be explained.

The input speech vector described above is given to the perceptualweighting filter 315 and after the weighting processing in considerationof human perceptual characteristics, the input speech vector is given toa subtracter 318 to be subtracted. Further, a zero input response vectorin relation to a synthesis filter 329, is given for input to subtracter318. Thus, a speech vector, from which an influence of the synthesisfilter 329 in the immediately before analysis frame is excluded, isgiven to subtracter 319 from subtractor 318. Further, a zero inputresponse vector in relation to the perceptual weighting filter 315, isgiven for input to subtracter 319. Thus, a speech vector, from which aninfluence of the weighted filter 315 in the immediately before analysisframe is obtained, is given to subtracter 330 from subtractor 319.

The optimum synthesized speech search portion 302 serves to search anexcitation source parameter in which the synthesis speech vector in thelocal reproduction is most similar to the target speech vector, and iscomposed of adaptive excitation codebook 320, stochastic excitationcodebook 321, pulse-like excitation codebook 322, VQ gain codebook 323,VQ gain controllers 324 and 327, adder 325, fixed codebook selectionswitch 326, code vector conversion circuit 328, synthesis filter 329,subtracter 330, error power sum calculation circuit 331 and codeselection circuit 332.

Each of the adaptive excitation codebook 320, stochastic excitationcodebook 321 and pulse-like excitation codebook 322 stores an adaptiveexcitation code vector, which is a waveform code in relation to anexcitation signal, stochastic excitation, code vector and pulse-likeexcitation, code vector, respectively, and VQ gain codebook 323 stores aVQ gain code which is related to an adaptive excitation code vector andfixed code vector (which generally represents stochastic excitation codevector and pulse-like excitation code vector).

The adaptive excitation code vector contributes to the voiced speechsignal having stochastic periodicity, while the stochastic excitationcode vector contributes to the unvoiced speech signal havingstochastically less periodicity. The adaptive excitation code vector theadaptive excitation codebook 320 is adaptively updated as describedpresently.

The pulse-like excitation code vector is a waveform excitation codevector consisting of unit impulse and is considered to contribute to thesteady portion of the voiced speech signal having a strong periodicity.

The VQ gain code is vector-quantized, for example, and one component ofthe vector relates to VQ gain for the adaptive excitation code vectorand the other component relates to VQ gain for the fixed code vector.

The pulse-like excitation code vector is a periodic simple signal whichcan be generated by means of a pulse signal generating circuit, but itcan preferably be generated by coding and reading out from the codebook322 as in this code excitation linear predictive encoder, the reason forwhich will be explained presently. Namely, it is easy to synchronize theexcitation vector with an output from the adaptive excitation codebook320. The same processing for selecting the stochastic excitationcodebook can be a pulse-like excitation code vector search byconstituting the excitation code vector to have the same codebookconstruction as the codebook 321.

By utilizing the various codebooks to obtain an optimum code, thelocally synthesized speech vector becomes the most similar to the targetspeech vector, and its indices are given to the multiplex circuit 303and are transmitted to the code excitation linear predictive decoderportion.

In case of the search of an optimum code including a selection of thestochastic excitation code vector or the pulse-like excitation codevector as described above, the searching is carried out with respect tothe adaptive excitation code, stochastic excitation code, pulse-likeexcitation code and VQ gain code, in turn, in this code excitationlinear predictive encoder.

In case of searching an optimum adaptive excitation code vector, anoutput from the stochastic excitation codebook 321 and the pulse-likeexcitation codebook 322 are assigned to be zero (0), and the VQ gaincontroller 324 multiplies a suitable value of a VQ coefficient ("1", forexample). In this state, the adaptive excitation codebook 320 outputsall of the stored adaptive excitation code vector sequentially or inparallel, and gives it as an excitation code vector to the synthesisfilter 329 through the VQ gain controller 324 and the adder 325. Thesynthesis filter 329 carries out a convolutional computing relative tothe excitation code vector, by utilizing, as a tap coefficient, the LPCwhich is given from the LPC conversion circuit 314, and synthesizedspeech vectors, which are synthesized only by the content of theadaptive excitation code vector as the excitation source signal, areobtained with respect to all the adaptive excitation code vector.

The subtracter 330 obtains, with respect to all of the adaptiveexcitation code vector, an error vector between the synthesized speechvector on which only the content of the adaptive.,excitation code vectoris effected and the target speech vector, and then gives it to the errorpower sum calculation circuit 331. The error power sum calculationcircuit 331 obtains a square sum (error power sum) of the error vector,with respect to all the adaptive code vector, and gives it to a codeselection circuit 332. The code selection circuit 332 determines theadaptive excitation code vector to minimize the error power sum.

Next, an optimum stochastic excitation code vector searching is carriedout and in the searching of this, a fixed codebook selection switch 326is driven to the side of the stochastic excitation codebook 321, theoutput from adaptive excitation codebook 320 is set to zero (0) or tothe previously obtained optimum adaptive excitation code vector. In thisstate, the stochastic excitation codebook 321 outputs sequentially or inparallel, all the stored stochastic excitation code vectors,and inputsthem into the code vector conversion circuit 328 through the fixedcodebook selection switch 326.

The code vector conversion circuit 328 proceeds with the conversion ofthe frequency characteristics of the inputted stochastic excitation codevector so that it is moved to close the frequency characteristics of aninput speech vector in correspondence with the time-length of thestochastic excitation code vector. As described above, all thestochastic exited code vector with its frequency characteristics beingconversion-processed is given, as an excitation code vector, to thesynthesis filter 329. Thereafter, it is processed similarly to thesearching of the optimum adaptive excitation code vector, and the codeselection circuit 332 determines an optimum stochastic excitation codevector.

After the searching of the optimum stochastic excitation code vector isfinished as described above, a searching of an optimum pulse-likeexcitation code vector is carried out. At this searching, the fixedcodebook selection switch 326 is driven to the side of the pulse-likeexcitation codebook 322 the output from adaptive excitation codebook 320is set to zero (0) or to the previously obtained optimum adaptiveexcitation code vector. In this state, the pulse-like excitationcodebook 322 outputs sequentially or in parallel, all the storedpulse-like excitation code vectors. Processings thereafter issubstantially similar to that of the moment when an optimum stochasticexcitation code vector is searched and, accordingly, a more detailedexplanation is not necessary.

As described above, when the optimum pulse-like excitation code vectoris determined, the code selection circuit 332 compares the error powersum of the selected code vector in the stochastic excitation code vectorsearch with the error power sum of the selected code vector in thepulse-like excitation code vector search to obtain smallest error powersum, and determine a fixed code to be transmitted to the code excitationlinear predictive decoder.

Thereafter, a searching of an optimum VQ gain code is carried out. Atthe searching of this VQ gain code, an optimum (selected) adaptiveexcitation code vector is transmitted from the adaptive excitationcodebook 320, and the fixed codebook selection switch 326 is switched toeither the selected stochastic excitation codebook 321 or pulse-likeexcitation codebook 322, and an optimum (selected) fixed code vector isoutputted from the selected fixed codebook 321 or 322. VQ gain codebook323 is composed of VQ gain for an adaptive excitation code vector and VQgain for the fixed code vector. The VQ gain for the adaptive excitationcode vector is given to the VQ gain controller 324 and the VQ gain forthe fixed code vector is given to the VQ gain controller 327. Thus, boththe VQ gain-controlled optimum adaptive excitation code vector and theoptimum fixed code vector, which have been processed with respect to afrequency characteristic operation and VQ gain control, are added by theadder 325 and then given to synthesis filter 329 as an excitation codevector. This processing is carried out sequentially or in parallel,relative to all the VQ gain codes in the VQ gain codebook 323.

After an optimum adaptive excitation code 303b, optimum fixed code 303 cand optimum VQ gain code 303e are selected, the code selection circuit332 gives the indexes, I_(s), I_(a) and I_(g) respectively of thesecodes to the multiplex circuit 303, and, fixed codebook selectionswitching information 303d, which provides information as to which oneof the stochastic excitation code vector and the pulse-like excitationcode vector is actually selected, is given to the multiplex circuit 303.The multiplex circuit 303 multiplexes the indexes with the LSPparameters given from the LSP parameter coding circuit 312 and transmitsthe coded speech information to the code excitation linear predictivedecoder. Incidentally, in the case of utilizing a vector quantizationfor a VQ gain coding method, the transmitted index is a vector number.

The coding processings described above is repeated with respect to eachsubframe, and the coded speech information is transmitted in turn to thecode excitation linear predictive decoder.

FIG. 5 shows in detail the specific structure of the code vectorconversion circuit 328. In FIG 5, the code vector conversion circuit 328has two cascaded filters 328a and 328b, and a pitch lag decision circuit328c.

The fixed code vector is given to a first filter 328a. An impulseresponse Hi(Z) of the first filter 328a is set as shown by formula (6),by which the frequency conversion processing is carried out relative tothe fixed vector.

    H1(Z)=(1-ΣA.sup.j ajZ.sup.-j)                        (6)

wherein aj (j is 1 to p) is a tap coefficient relative to synthesisfilter 329 which is supplied from the LPC conversion circuit 314, and pis a vocal tract analysis order. Further, A and B are constants whichare determined in the ranges of 0<A≦1, and 0<B≦1.

The code vector which was processed in its frequency characteristics bythe first filter 328a is transmitted to the second filter 328b. Thepitch lag decision circuit 328c obtains a pitch lag L from the index ofthe optimum adaptive excitation code relative to the adaptive excitationcodebook 320 and then gives the pitch lag L to the second filter 328b.An impulse response H2(Z) of the second filter 328b is determined asshown by formula (7), by which a frequency conversion is carried outrelative to the inputted fixed code vector.

    H2(Z)=1/(1εZ.sup.-L)                               (7)

wherein ε is a constant determined in the range of 0<ε≦1. An output ofthe second filter 328b is given to VQ gain controller 327 shown in FIG.3.

By the code vector conversion circuit 328 as described above, thefrequency characteristics of inputted fixed code vector can be madecloser to the frequency characteristics of the input speech vector, inaccordance with the time length of the fixed code vector.

Accordingly, the code excited linear predictive coding apparatus(encoder) can provide a high quality regenerated speech signal.

Next, a code excitation linear predictive decoder in correspondence withthe code excitation linear predictive coding apparatus (encoder) shownin FIG. 3 will be described with reference to the accompanying drawing.

FIG. 4 is a block diagram of a code excitation linear predictive decoderwhich corresponds to the code excitation linear predictive codingapparatus (encoder) shown in FIG. 3. In FIG. 4, the code excitationlinear predictive decoder has demultiplex circuit 440, LSP parameterdecoding circuit 441, LPC coefficient conversion circuit 442, adaptiveexcitation codebook 443, stochastic excitation codebook 444, pulse-likeexcitation codebook 445, VQ gain codebook 446, VQ gain controller 447,VQ gain controller 449, fixed codebook selection switch 448, code vectorconversion circuit 450, adder 451 and synthesis filter 452.

The coded speech information given from the code excitation linearpredictive encoder is input to the demultiplex circuit 440. Thedemultiplex circuit 440 separates the coded speech information into LSPparameter code, index of the optimum adaptive excitation code, index ofthe optimum fixed code, index of the optimum VQ gain codebook and fixedcode selection switch information.

Then, LSP parameter code is given to the LSP parameter decoding circuit441 and the index of the optimum adaptive excitation code is given tothe adaptive excitation codebook 443. Further, the index of optimum VQgain code is given to the VQ gain codebook 446 and the fixed codebookselection switch information is given to the fixed codebook selectionswitch 448.

The index of the optimum fixed code is given to pulse-like excitationcodebook 445 or a stochastic excitation codebook 444 which aredetermined by the fixed code selection switching information. Theadaptive excitation codebook 443 outputs an adaptive excitation codevector which is determined by a given index, and this adaptiveexcitation code vector is VQ gain-controlled through VQ gain controller447 and given to an adder 451. Further, the adaptive excitation codebook443 gives an adaptive excitation code vector to a code vector conversioncircuit 450.

The stochastic excitation codebook 444 or pulse-like excitation codebook445 gives a stochastic excitation code vector or pulse-like excitationcode vector, which corresponds to the given index, to a code vectorconversion circuit 450 through a fixed codebook selection switch 448.

The code vector conversion circuit 450 operates so that the frequencycharacteristics become closer to the frequency characteristics of theinput speech vector in accordance with the index of the LPC and adaptiveexcitation code vector. A specific structure of the code vectorconversion circuit 450 is the same as that of the structure shown inFIG. 5. Thus, the frequency-processed fixed code vector is VQgain-controlled by a VQ gain controller 449 and then given to an adder451.

The adder 451 adds the given adaptive excitation code vector and thefixed code vector together, and the added vector is assigned to be anexcitation code vector, which is then given to the synthesis filter 452.The synthesis filter 452 outputs a synthesized speech vector.

The code excitation linear predictive decoder conducts theabove-described processes every time a decoded speech vector is givenor, in other words, for each subframe.

Important features of the present invention are that the LSP parameteris used and transmitted as a vocal tract parameter; a pulse-likeexcitation codebook is provided for giving an excitation sourceparameter; and a frequency characteristic of the fixed code vector iscontrolled. These features can be independently provided to each of thecoding apparatus and decoding apparatus without failure of theadvantages and effects thereof.

In addition, the coding apparatus and decoding apparatus described aboveare related primarily to the forward-type code excitation linearpredictive encoder and decoder, respectively, but the present inventionis not limited thereto but applicable to a backward-type code excitationlinear predictive encoder and decoder, respectively.

The above-described encoder and decoder were intentionally designedunder the technological basis for seeking to solve the problems inducedfrom the low rate coding of 4-bit/s or less. However, more favorablesound reproduction can be realized if they are adapted to encoders anddecoders having coding at a higher rate. If the higher coding rate isallowable, both of the stochastic excitation codebook and pulse-likeexcitation codebook can cooperate effectively rather than selectivelyoperating either the stochastic excitation codebook or the pulse-likeexcitation codebook.

INDUSTRIAL APPLICABILITY

According to the present invention, it is considered that the frequencycharacteristic of an actual excitation code vector is relatively closeto that of an input speech vector and, in order to make the frequency ofthe excitation code vector closer to a frequency of the input speechvector, the stochastic excitation code vector is convolutionallycomputed utilizing a specific impulse response. Thereafter, an adaptiveexcitation code vector is added to produce an excitation code vectorand, therefore, an excitation code vector which is well adapted to aninput speech vector by a small number of vector vectors can be obtainedand, at the same time, the quantization error can be masked with aconversion operation of an excitation code vector, thereby improvingreproduction quality.

Further, in addition to the adaptive excitation codebook and stochasticexcitation codebook, pulse-like excitation codebook is disposed whichstores therein a pulse-like excitation code vector composed a unitimpulse and, accordingly, rapid tracking of a speech signal havingperiodicity can be realized, and a clear pulse-like excitation codevector can be formed at a steady portion of the speech signal.

Besides, since the pulse-like excitation code vector and the stochasticexcitation code vector are switched over, the apparatus of the presentinvention can be adapted to low rate coding, and a favorably reproducedspeech can be realized at the time, for example, of a transitionalperiod of the speech in which there are random signals and pulse-likesignals together.

In addition, according to the code excitation linear coding apparatusand decoding apparatus, an excitation code vector is selected and usedfrom either a stochastic excitation codebook or a pulse-like excitationcodebook and, therefore, a favorable reproduction of speech sound can berealized with the condition that the number of coded bits of theexcitation source parameter is small.

Further, the vocal tract parameter for sound synthesization is used asan LSP parameter which gives less distortion to the vocal tract vectorthan LPC when it is coded with a smaller number of code bits and,therefore, reproduction quality at a lower coding rate can be improvedfrom a vocal tract parameter viewpoint.

What is claimed is:
 1. A code excitation linear predictive codingapparatus comprising:excitation codebook means for selectivelyoutputting an excitation code vector as an excitation source informationof a speech signal; and code vector conversion circuit means forconverting the excitation code vector selectively output from theexcitation codebook means into a frequency characteristic determined atthe time of output of said excitation code vector.
 2. A coding apparatusaccording to claim 1, wherein the code vector conversion circuit meansgenerates an impulse response of a transfer function which is determinedin accordance with a vocal tract parameter of an input speech signal,and convolves the excitation code vector with the impulse response.
 3. Acoding apparatus according to claim 2, wherein the impulse response ofthe transfer function which is determined in accordance with the vocaltract parameter is represented by:

    H(Z)=(1-ΣA.sup.j ajZ.sup.-j)/(1ΣB.sup.j ajZ.sup.-j)

where aj (j is 1 to p) is a linear predictive coefficient; p is a vocaltract analysis order; and A and B are in the ranges:0<A<1 and 0<B1.
 4. Acoding apparatus according to claim 1, wherein the code vectorconversion circuit means generates an impulse response of a transferfunction which is determined in accordance with an excited pitch lag,and convolves the excitation code vector with the impulse response.
 5. Acoding apparatus according to claim 4, wherein the impulse response ofthe transfer function which is determined in accordance with the excitedpitch lag is represented by:

    H(Z)=1/(1-εZ.sup.-L)

where ε is a constant satisfying a range of 0<ε≦1; and L is a pitch lagsignal.
 6. A coding apparatus according to claim 1, wherein the codevector conversion circuit means convolves the excitation code vectorwith the impulse response of the transfer function which is determinedin accordance with transfer functions represented by:

    H(Z)=(1-ΣA.sup.j ajZ.sup.-j)/(1-ΣB.sup.j ajZ.sup.-j)

and

    H(Z)=1/(1-εZ.sup.-L)

where aj (j is 1 to p) is a linear predictive coefficient; p is a vocaltract analysis order; A, B and ε are in the ranges: 0<A<1, 0<B<1 and0<ε≦1; and L is a pitch lag signal.
 7. A code excitation linearpredictive decoding apparatus comprising:excitation codebook means forselectively outputting an excitation code vector as an excitation sourceinformation of a speech signal; and code vector conversion circuit meansfor converting the excitation code vector selectively output from theexcitation codebook into a frequency characteristic determined at thetime of output of said excitation code vector.
 8. A decoding apparatusaccording to claim 7, wherein the code vector conversion circuit meansgenerates an impulse response of a transfer function which is determinedin accordance with a vocal tract parameter of an input speech signal,and convolves the excitation code vector with the impulse response.
 9. Adecoding apparatus according to claim 8, wherein the impulse response ofthe transfer function which is determined in accordance with the vocaltract parameter is represented by:

    H(Z)=(1-εA.sup.j ajZ.sup.-j)/(1-εB.sup.j ajZ.sup.-j)

where aj (j is 1 to p) is a linear predictive coefficient; p is a vocaltract analysis order; and A and B are in the ranges: 0<A<1 and 0<B1. 10.A decoding apparatus according to claim 7, wherein the code vectorconversion circuit means generates an impulse response of a transferfunction which is determined in accordance with an excited pitch lag,and convolves the excitation code vector with the impulse response. 11.A coding apparatus according to claim 10, wherein the impulse responseof the transfer function which is determined in accordance with theexcited pitch lag is represented by:

    H(Z)=1/(1εZ.sup.31 L)

where ε is a constant satisfying a range of 0<ε≦1; and L is a pitch lagsignal.
 12. A decoding apparatus according to claim 7, wherein the codevector conversion circuit means convolves the excitation code vectorwith the impulse response of the transfer function which is determinedin accordance with transfer functions represented by:

    H(Z)=(1-εA.sup.j ajZ.sup.-j)/(1-εB.sup.j ajZ.sup.-j)

and

    H(Z)=1/(1-εZ.sup.-L)

where aj(j is 1 to p) is a linear predictive coefficient; p is a vocaltract analysis order; A, B and ε are in the ranges: 0<A<1, 0<B<1 and0<ε≦1; and L is a pitch lag signal.
 13. A code excitation linearpredictive coding apparatus comprising:excitation codebook means foroutputting an excitation code vector as an excitation source informationof a speech signal; and pulse-like excitation codebook means for storinga pulse-like excitation code vector composed of an unit impulse.
 14. Acode excitation linear predictive coding apparatus according to claim13, further comprising means for generating a pulse-like excitation codevector from the pulse-like excitation codebook means, and means fortransmitting information indicative of what pulse-like excitation codevector is selected to a code excitation linear predictive decodingapparatus.
 15. A code excitation linear predictive coding or decodingapparatus according to claim 14, further comprising:code vectorconversion circuit means for converting the pulse-like excitation codevector transmitted from the pulse-like excitation codebook into afrequency characteristic determined at the time of output of thepulse-like excitation code vector.
 16. A code excitation linearpredictive coding apparatus according to claim 13, further comprisingmeans for generating a vocal tract parameter, and transmitting the vocaltract parameter in the form of a linear spectrum pair parameter to acode excitation linear predictive decoding apparatus.
 17. A codeexcitation linear predictive decoding apparatus comprising:excitationcodebook means for outputting an excitation code vector as an excitationsource information of a speech signal; and pulse-like excitationcodebook means for storing a pulse-like excitation code vector composedof an unit impulse.
 18. A code excitation linear predictive decodingapparatus according to claim 17, further comprising means for selectingthe pulse-like excitation code vector in the pulse-like excitationcodebook in accordance with selected information transmitted from acorresponding code excitation linear predictive coding apparatus.
 19. Acode excitation linear predictive decoding apparatus according to claim17, further comprising means for receiving a vocal tract parameter inthe form of a linear spectrum pair parameter used for vocal tractreproduction from a corresponding code excitation linear predictivecoding apparatus.
 20. A code excitation linear predictive coding methodcomprising the steps of:selectively outputting an excitation code vectorfrom an excitation codebook as an excitation source information of aspeech signal; converting the excitation code vector into a convertedcode vector having a frequency characteristic; and multiplying theconverted code vector by a gain output from a gain codebook.
 21. A codeexcitation linear predictive coding method according to claim 20,wherein the converting step comprises the steps of:generating an impulseresponse of a transfer function which is determined in accordance with avocal tract parameter output from an input speech vector; and convolvingthe excitation code signal with the impulse response in order to obtainthe converted code vector.
 22. A code excitation linear predictivecoding method according to claim 20, wherein the converting stepcomprises the steps of:generating an impulse response of a transferfunction which is determined in accordance with an excited pitch lagobtained from indexes of an adaptive excitation code; and convolving theexcitation code signal with the impulse response in order to obtain theconverted code vector.